In a variable speed system of secondary excitation, a frequency converter operates with a low-frequency output current. Since the low-frequency output current near a synchronous speed becomes a very low frequency current, a time during which a current near to a peak value flows in an element prolongs. For this reason, the element is overheated. In the worst case, the element may fail. In particular, the current is DC current at a synchronous speed. When the peak current continuously flows in the element, the operation condition is severer than that at the very low frequency.
As a method of operating the variable speed system of secondary excitation near the synchronous speed, a method of monitoring the temperature of an element, stopping the frequency converter by a protection circuit at a high temperature, and controlling the operation so as not to stay the system in a speed forbidden zone near the synchronous speed is available.
As shown in FIG. 10, a conventional variable speed system of secondary excitation including a variable speed control apparatus includes, as main components, a double feed synchronous machine 1 mechanically connected to a motor, a generator-side breaker 2 connected to the primary side of the double feed synchronous machine 1, a main voltage transformer 3 capable of performing voltage regulation by tap switching, a frequency converter 4 for supplying a current to the secondary winding of the double feed synchronous machine 1, and a motor 5 directly connected to the rotating shaft of the double feed synchronous machine 1. In addition, the variable speed system includes, as various kinds of control devices, a motor control device 6, a secondary current controller 7, a speed controller 8, a speed forbidden zone prevention controller 9, an effective power controller 10, and a voltage controller 11. The variable speed system further includes, as various kinds of detectors, a voltage detector 12, a phase detector 13, a secondary current detector 14, a rotation speed detector 15, a primary frequency detector 16, and an effective power detector 17.
The voltage detector 12 detects a primary voltage or system voltage VL of the double feed synchronous machine 1. The phase detector 13 detects a rotation phase θR of the double feed synchronous machine 1. The secondary current detector 14 detects a secondary current iI of the double feed synchronous machine 1. The rotation speed detector 15 detects a rotation speed ω of the double feed synchronous machine 1. The primary frequency detector 16 detects a primary frequency FL from the primary voltage VL. The effective power detector 17 detects an effective power from the primary side of the double feed synchronous machine 1.
The effective power controller 10 determines a target speed value ω0* from the effective power and the primary frequency FL and outputs the target speed value. The voltage controller 11 determines a reactive component current command Id* from the primary voltage VL or the like, and outputs the reactive component current command Id*. The speed forbidden zone prevention controller 9 adjusts a speed command value ω* in accordance with whether the target speed value ω0* passes through the speed forbidden zone and outputs the speed command value ω0*. The speed controller 8 determines an effective component current command Iq* of the secondary current from the rotation speed ω and the speed command value ω* of the double feed synchronous machine 1, and outputs the effective component current command Iq* of the secondary current. The secondary current controller 7 generates a gate pulse to be output to the frequency converter 4, based on the effective component current command Iq*, the reactive component current command Id*, the secondary current iI, the primary voltage VL, and the rotation phase θR and controls the output current of the frequency converter 4 based on the gate pulse. The motor control device 5 controls the mechanical input/output of the motor 5.
The secondary current controller 7 includes a phase reference operation unit 71, an effective and reactive component operation unit 72, subtractors 73 and 75, controllers 74 and 76, an output voltage operation unit 77, a triangular wave generator 78, a gate pulse generator 79, and an AND circuit 7A, as shown in FIG. 11.
The phase reference operation unit 71 calculates a converter current phase reference θI0 from the primary voltage VL detected by the voltage detector 12 and the rotation phase θR detected by the phase detector 13. The effective and reactive component operation unit 72 calculates an effective component current Iq and a reactive component current Id from the converter current phase reference θI0 and the secondary current detected by the current detector 14. The subtractor 73 obtains a deviation between the effective component current command Iq* output from the speed controller 8 and the effective component current Iq and sends this deviation to the controller 74. Similarly, the subtractor 75 obtains a deviation between the reactive component current command Id* output from the voltage controller 11 and the reactive component current Id and sends this deviation to the controller 76. The output voltage operation unit 77 calculates an output voltage VI from an effective component output voltage Vq* output from the controller 74, a reactive component output voltage Vd* output from the controller 76, and the converter current phase reference θI0. The triangular wave generator 78 generates a triangular wave CRY based on an oscillation frequency OCS. The gate pulse generator 79 outputs a gate pulse to the frequency converter 4 via the AND circuit 7A at a timing determined by an intersection between the output voltage VI and the triangular wave CRY. Note that when the AND circuit 7A receives a gate block (GB) signal, the circuit 7A blocks the gate pulse to stop the frequency converter 4.
The speed controller 8 includes a subtractor 81 and a controller 82, as shown in FIG. 12.
The subtractor 81 obtains a deviation between the speed command value ω* output from the speed forbidden zone prevention controller 9 and the rotation speed ω detected by the rotation speed detector 15 and sends this deviation to the controller 82. Upon reception of the deviation obtained by the subtractor 81, the controller 82 sends an output 8a as the effective component current command. Iq* to the secondary current controller 7.
The speed forbidden zone prevention controller 9 includes a hysteresis function means 91 and a change rate limiter 92, as shown in FIG. 13.
Upon reception of the target speed value ω0* from the effective power controller 10, the hysteresis function means 91 sends an output 9a to the change rate limiter 92. The change rate limiter 92 sends an output 9b as the speed command value ω* to the speed controller 8. Note that the relationship between the input ω0* and an output 91a of the hysteresis function means 91 is shown in FIG. 14. That is,
Assuming that ω0* increases,if ω0*<ω′L or ω′U<ω0*, then ω*=ω0*,if ω′≦ω0*≦ω′U, then ω*=ω′L 
Assuming that ω0* decreases,if ω0*<ω′L or ω′U<ω0*, then ω*=ω0*,if ω′L≦ω0*≦ω′U, then ω*=ω′U 
FIG. 15 shows the characteristic of the speed command value ω* by the above method. Even if the target speed value ω0* passes through the speed forbidden zone from time t1 to time t2, the speed command value ω* waits at a forbidden zone lower limit speed until time t2 and passes through the forbidden zone within a short period of time from time t2 to time t3.
In the related art, the speed command value quickly passes through the speed forbidden zone to reduce the load on the element while the speed command value passes through the speed forbidden zone. However, when the shaft input/output of the motor increases as in adjacent machine breaking, it is not inevitably prevent an increase in output current of the frequency converter under the above control. The element may damage. It is also proposed that the load of the element is reduced while the speed command value passes through the speed forbidden zone by narrowing down the converter current. However, when the current is narrowed down, the forbidden zone passing time prolongs to increase the load of the element. The element may damage.
Under these circumstances, it is desired to provide a variable speed control apparatus and its operation method capable of continuing safety operation without overloading the element of the frequency converter even if a disturbance or the like occurs.